The effect of hind-tibial spurs on digging rate in female eastern cicada killers

Abstract.  1. The function of the large hind-tibial spurs of female eastern cicada killer wasps ( Sphecius speciosus ), thought to be adaptations to digging, was examined by measuring the effect of spurs on digging rate.
2. Spurs were removed from one group of wasps and left intact in control specimens.
3. Wasps with intact spurs removed soil from the burrow at a rate of 0.98 ± 0.11 (16) g dry mass min⁻¹ mean ± SEM ( N )]. Removal of spurs results in an average digging rate just over one-half that, 0.54 ± 0.13 (9) g min⁻¹.
4. The difference in digging rate was primarily because of a twofold greater mass of soil unloaded by intact wasps each time they exited the burrow, rather than the rate at which such loads were delivered.
5. Such inefficiency results in an estimated additional 8.2 h requirement to fully excavate an average burrow, or a 1–4% reduction in time available for foraging in spur-ablated wasps.
6. Calculations suggest that the time lost to digging would result in a 3.9–19.5% reduction in foraging success for spurless females.
7. These temporal and resulting foraging advantages of hind-tibial spurs presumably provided selection pressure for their evolution from smaller, pre-existing setae.